Pulsed leakage tagging signal

ABSTRACT

A method of transmitting a leakage tagging signal includes inserting a tagging signal into a CATV television signal only during portions of the television signal in which control information is present. A method according to the present invention is a method of transmitting a tagging signal in a communication system, the communication system transmitting television signals comprising control information and program information. One step of the method includes generating a tagging signal that is detectable by corresponding leakage detecting equipment. The method also includes the step of identifying a portion of a television signal in which control information is present. Finally, the method of the present invention encompasses inserting the tagging signal into the television signal only during the portion of the television signal in which control information is present.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation under 35 U.S.C. §120 of U.S. patent applicationSer. No. 08/943,652, filed Oct. 3, 1997 now U.S. Pat. No. 6,307,593.

FIELD OF THE INVENTION

The present invention relates generally to the field of cable television(CATV) transmission system testing, and in particular, to leakagetesting in CATV transmission systems.

BACKGROUND OF THE INVENTION

Cable television systems, or CATV systems, are used in a widespreadmanner for the transmission and distribution of television signals toend users, or subscribers. In general, CATV systems comprise atransmission subsystem and a distribution subsystem. The transmissionsubsystem obtains television signals associated with a plurality of CATVchannels and generates a broadband CATV signal therefrom. Thedistribution subsystem then delivers the CATV broadband signal totelevision receivers located within the residences and businessestablishments of subscribers.

One problem facing CATV service providers is signal leakage. Signalleakage refers to the transmission and/or reception of signals throughbreaches or other nonconformities in the CATV distribution subsystem. Inparticular, the distribution subsystem, which typically comprisescoaxial cable, amplifiers and other devices, ideally provides arelatively low-loss conduit between the CATV transmission subsystem andsubscribers' television receivers. If, however, portions of thedistribution subsystem are physically damaged, for example, the coaxialcable is damaged, kinked or broken, then CATV signals may leak throughthe damaged portions, causing unwanted transmission into the atmosphere.

The primary problem associated with the transmission of the CATV signalinto the atmosphere via leakage is potential interference withaeronautical communications. Portions of the allocated CATV bandwidthoverlap with frequencies allocated for aeronautical communication.Excessive leakage of CATV signals can therefore undesirably interferewith aeronautical-related signal transmission and reception.

Another problem caused by excessive leakage arises in the context ofreverse path communication. Reverse path communication refers tocommunication signals generated by CATV subscribers and transmitted tothe CATV transmission subsystem. The problem caused by leakage is thatsignal ingress due to leakage can undesirably interfere with suchreverse path communications. Signal ingress is the infiltration ofspurious external signals into the CATV distribution subsystem. If thereare several leakage points in the distribution subsystem, then thespurious signals from those several leakage points in the CATVdistribution subsystem will tend to accumulate at the CATV transmissionsubsystem. The accumulated spurious signals can have considerableenergy, thereby potentially causing interference with reverse pathcommunication signals.

As a result, signal leakage is an undesirable phenomenon that CATVservice providers strive to reduce.

In order to reduce leakage, CATV service providers must first determinethe location of leakage points in the distribution subsystem. Variousleakage detection devices are currently available that assist in thelocation of leakage points. Such devices typically include an antennaand a receiver that is tuned to a particular frequency in the CATVsignal bandwidth. The detector further includes a signal strengthmeasurement circuit. To detect leakage, a technician typically drivesalong a route that traces a portion of the CATV distribution system,preferably in the vicinity of a suspected leakage location. If thesignal strength measurement circuit detects a relatively large amplitudesignal at a particular location, then a leak may be indicated in or nearthat location. The technician may then use the leakage detector topinpoint the source of the leak. Once the source of the leak ispinpointed, corrective action maybe taken.

A drawback of the above described leakage detection devices is theirinability to distinguish CATV signals leaked from the system under testfrom other signals in the same bandwidth. This drawback is becoming ofincreasing importance due to the proliferation of CATV serviceproviders. In particular, two or more CATV service providers often haveportions of their distribution systems that overlap, or at least aredisposed in close proximity to one another. As a result, when atechnician detects leakage in a particular location, that leakage mayeither be caused by the system under test or by another system. BecauseCATV service providers are primarily interested only in leakage in theirown distribution system, it is desirable to ascertain the identity ofthe source of the leak.

One prior art method of addressing the problem of differentiating theleakage signals from a system under test from other signals is describedin U.S. Pat. No. 4,237,486 to Shimp, issued Dec. 2, 1980. Shimpdescribes a method of modulating a distinctive tagging signal on anunused CATV channel frequency at the transmission subsystem. The leakagedetector is then tuned to that CATV channel frequency and used forleakage detection. If a relatively large signal strength is detected,the leakage detector then attempts to isolate or detect the distinctivetagging signal. If the tagging signal cannot be detected, then it isdetermined that the detected signals are not caused by leakage in thesystem under test. If, however, the tagging signal is detected, then itis determined that the detected signal is due to leakage in the systemunder test.

While the Shimp patent discusses a method of determining whetherdetected signals in the bandwidth of interest are caused by leakage bythe system under test, that method requires an unused channel frequency.The requirement that an unused frequency be used undesirably consumesvaluable CATV bandwidth.

U.S. Pat. No. 5,608,428 to Bush proposes a system in which a taggingsignal is modulated onto an active video carrier, in other words, anin-use channel frequency. Specifically, a low frequency oscillatingsignal is modulated onto the active video carrier. That low frequencyoscillating signal may then be detected by a leakage detector todetermine whether a detected signal is caused by leakage by the systemunder test. The low frequency signal is chosen such that automatic gaincontrol (“AGC”) circuit in many subscribers' television receivers willreduce the interference caused by the low frequency signal. While thesystem proposed by Bush does not require an out of service channel fortagged leakage detection, the modulation of the low frequency signalnevertheless can cause undesirable signal distortion to somesubscribers.

Accordingly, there is a need for a leakage tagging method and apparatusthat does not require an out of use channel, but also has a reducedlikelihood of producing distortion in the used channels of a CATVsignal.

SUMMARY OF THE INVENTION

The present invention fulfills the above need, as well as others, byproviding a leakage tagging method that inserts a tagging signal into atelevision signal only during times in which control information ispresent in the television signal. The control information in atelevision signal includes, for example, the vertical synchronization(“sync”) information, the horizontal sync information, and potentialquiet lines. By inserting the tagging signal only during times in whichcontrol information is present, the tagging signal generally does notinterfere with the program information, in other words, the picture andsound information. As a result, the tagging signal of the presentinvention provides little or no perceptible distortion to the end user.

An exemplary method according to the present invention is a method oftransmitting a tagging signal in a communication system, thecommunication system transmitting television signals comprising controlinformation and program information. One step of the method includesgenerating a tagging signal that is detectable by corresponding leakagedetecting equipment. The method also includes the step of identifying aportion of a television signal in which control information is present.Finally, the method of the present invention encompasses inserting thetagging signal into the television signal only during the portion of thetelevision signal in which control information is present.

Optionally, the above exemplary method further includes inserting thetagging signal by first modulating a carrier signal with the taggingsignal and summing the carrier signal modulated with the tagging signalwith the television signal. In such an embodiment, it is preferably touse a carrier signal that has a frequency within the vestigial sidebandof a CATV channel frequency. The vestigial sideband is a frequency bandlocated below the carrier frequency associated with the CATV channel ofthe television signal, but above the frequency band used by the adjacentCATV channel.

The present invention further includes various apparatus for carryingout the above method. For example, one apparatus according to thepresent invention includes an arrangement for the insertion of a taggingsignal into a television signal, the television signal comprisingcontrol information and program information, the television signal beingtransmitted by a communication system. Such an arrangement comprises aninput, a tagging signal generator, a signal inserter, and a switch.Specifically, the input is connected to a source of television signals.The tagging signal generator is operable to generate a tagging signalthat is detectable by corresponding leakage detecting equipment. Thesignal inserter, which is coupled to the input and the tagging signalgenerator, is operable to insert the tagging signal into the televisionsignal. The switch is operably coupled to the signal inserter to causethe signal inserter to insert the tagging signal only during the portionof the television signal in which control information is present.

The above features and advantages, as well as others, will become morereadily apparent to those of ordinary skill in the art by reference tothe following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a communication system including the an arrangement forinserting a tagging signal according to the present invention;

FIG. 2 shows an exemplary tag insertion arrangement according to thepresent invention;

FIG. 3 shows a signal timing diagram of a vertical interval in astandard NTSC baseband television signal;

FIG. 4 shows an exemplary leakage detector for use in detecting leakagesignals tagged in accordance with the present invention;

FIG. 5 shows a pulse peak detection circuit for use in the leakagedetector of FIG. 4.; and

FIG. 6 shows an alternative tag insertion arrangement according to thepresent invention.

DETAILED DESCRIPTION

FIG. 1 shows a communication system including the an arrangement forinserting a tagging signal according to the present invention. In thepresent embodiment, the communication system is a CATV system 10 fortransmitting and distributing television signals and other informationto subscriber reception devices. The CATV system 10 in this embodimenttransmits and distributes television signals in the NTSC standardformat, which is well known to those of ordinary skill in the art. TheCATV system 10 includes a head end transmission subsystem 12, adistribution subsystem 14, and a plurality of subscriber receivers shownby example herein as television receivers 16 ₁, 16 ₂, . . . 16 _(N).Subscriber receivers may alternatively include, among other things,video monitors and computer monitors.

The transmission subsystem 12 includes a plurality of sources oftelevision signals, shown by example herein as modulated video sources18 ₁, 18 ₂, . . . 18 _(M), and further includes a tag insertionarrangement 20 and a combiner 24. The transmission subsystem 12 isoperable to generate a CATV signal as is known in the art, with theexception that a tagging signal is inserted according to the presentinvention. In general, a CATV signal as described herein includes, amongother things, at least one television signal, the television signalincluding a baseband television signal modulated onto a carrier signal.Each carrier signal has a frequency associated with one of a pluralityof CATV channels.

The modulated video source 18 ₁ is a circuit well known in the art thatincludes a carrier signal generator, not shown, and a basebandtelevision signal source, not shown. The modulated video source isoperable to generate a television signal which comprises a carriersignal modulated by a baseband television signal. Likewise, themodulated video sources 18 ₂ . . . 18 _(M) are operable to generatetelevision signals which comprise carrier signals modulated bytelevision baseband signals.

Each modulated video source 18 _(x) is typically associated with one ofa plurality of CATV channels. Specifically, the television signalcarrier frequency, or CATV channel frequency, corresponds to anassociated CATV channel. The CATV channel frequencies are typicallywithin the 5 MHz to 1000 MHz frequency band, and are separated by apredetermined frequency interval. In United States cable systems, theCATV channel frequencies are typically separated by 6 MHz or integermultiples thereof.

The modulated video sources 18 ₁, 18 ₂, . . . 18 _(M) are each connectedto the signal combiner 24, which combines the various television signalsinto a single broadband CATV signal. According to the exemplaryembodiment of the present invention described herein, the modulatedvideo source 18 _(M) is connected to the signal combiner 24 through thetag insertion arrangement 20.

The tag insertion arrangement 20 is a circuit that is operable togenerate a tagging signal that is detectable by corresponding leakagedetecting equipment. A tagging signal is a signal having somesource-identifying information that may be recognized by the leakagedetecting equipment. For example, in the present embodiment, the taggingsignal includes a 20-29 Hz sine wave component. The leakage detectingequipment, shown here by exemplary leakage detector 21, is operable toisolate or detect that sine wave component, thereby confirming thesource of detected signals. Such confirmation of source allows theleakage detecting equipment to distinguish leakage signals of the systemunder test from either spurious signals of unknown origin or leakagesignals of a co-located CATV system.

In accordance with one embodiment of the present invention, the taginsertion arrangement 20 is operable to receive the television signalfrom the modulated video source 18 _(M) and identify portions oftelevision signal in which the baseband television signal contains onlycontrol information. Specifically, a baseband television signal has astandard format that includes program information and controlinformation. In most systems, the program information consists of theimage information, such as chrominance and luminance information, andaudio information. By contrast, the control information in general, doesnot include such image or audio content. The control information insteadcomprises the horizontal and vertical sync information, and may furtherinclude so-called quiet lines. An important aspect in the controlinformation is that it follows a repeating pattern for each frame ofvideo information. For example, the vertical sync information of a NTSCtelevision signal, referred to as the vertical interval, occurs twiceevery frame, or at a frequency of approximately 60 Hz. Horizontal syncinformation and quiet lines also occur at regular repeating intervals.

The tagging signal generator 20 is thus operable to identify therepeating occurrences of at least one type of control information, forexample, the vertical interval, within the television signal. Thetagging signal generator 20 is further operable to insert the taggingsignal into the television signal during portions of the televisionsignal when control information is present.

In the preferred embodiment described herein, the tag insertionarrangement 20 is operable to perform such insertion by first modulatingthe tagging signal onto a carrier signal having a frequency within thevestigial sideband associated with the television signal. The carriersignal modulated with the tagging signal is then combined with thetelevision signal. As will be discussed further below, use of thevestigial sideband carrier signal for the tagging signal further reducesthe potential for interference that is perceptible by subscribers.

FIG. 2, discussed further below, shows an exemplary embodiment of taginsertion arrangement according to the present invention having thecapabilities discussed above.

Continuing with the discussion of FIG. 1, the combiner 24 is connectedto provide CATV signals to the distribution system 14. The distributionsystem 14 typically comprises a plurality of elements including coaxialcable, repeater amplifiers, splitters and other elements typicallyemployed by CATV service providers. By way of representativeillustration only, the distribution system 14 is shown herein asincluding a cable 26 and a splitter 28. The cable 26 typically comprisesa network of coaxial cable or other suitable conduit for transmission ofCATV signals through a geographical area interspersed with subscribers.

In the example discussed herein, the cable 26 connects the combiner 24to the splitter 28. The splitter 28 is then connected to the pluralityof television receivers 16 ₁ . . . 16 _(N). The television receivers 16₁ . . . 16 _(N) are, in general, commercially available televisionreceivers designed or adapted to receive CATV signals and tune toparticular channels within the CATV signal. While the performance ofvarious types of televisions differ somewhat, substantially allcommercially available televisions are designed to include tuning andfiltering equipment having a minimum standard performance criteria.

Regardless of the particular make-up, the distribution system 14 issusceptible to faults, cable breaches, faulty interconnections, andother nonconformities that allow leakage. Leakage refers to both egressof CATV signals from the distribution system 14 and ingress of spurioussignals into the CATV system. The egress of the CATV signal out of thedistribution system 14 can potentially interfere with aeronauticalcommunication and ingress of spurious signals increases the noise withinthe CATV system.

The tag insertion arrangement 20 of the present invention generates aunique tagging signal that facilitates the detection of such leaks. Anexemplary operation of the tag insertion method and apparatus within theCATV system 10 is discussed herebelow.

In general, the plurality of the modulated video sources 18 ₁ . . . 18_(M) generate television signals associated with one of a plurality ofCATV channels. As discussed above, the baseband television signalcomprises program information, in other words, information related toimage and audio content, and control information, such assynchronization information. The format of a television baseband signalis standardized such that the control information is substantiallysimilar on all television baseband signals. For the purposes of thisdescription, control information includes the horizontal synchronizationpulses, the vertical interval, and quiet lines that may or may not beused to transmit other non-image related information.

Each of the modulated video sources 18 ₁ . . . 18 _(M) further generatesa vestigial sideband related to each carrier signal. In particular, whena baseband television signal is modulated onto a CATV channel carriersignal, two mirror image frequency spectra, referred to as sidebands,are produced. Thus, for example, if the baseband television spectrum isassumed to be approximately 4.5 MHz, then the modulation of the signalonto the CATV channel carrier signal produces sidebands extending ±4.5MHz from the CATV channel frequency. Because only one sideband isnecessary to reproduce the signal, one of the sidebands is filtered outto conserve bandwidth. In particular, the sideband that extends from theCATV channel frequency to 4.5 MHz below the CATV channel frequency issubstantially filtered out to allow use of that bandwidth by theadjacent CATV channel. A portion of the filtered spectrum, however, isnot used by the adjacent CATV channel. That portion is referred to asthe vestigial sideband of television signal. In United States CATVsystems, the vestigial sideband is typically up to 1.5 MHz below theCATV channel frequency. Accordingly, each of the modulated video sources18 ₁ . . . 18 _(M) generates dual sidebands and then through filtering,substantially attenuates one of the sidebands, thereby creating avestigial sideband.

The operation of the modulated video carrier 18 _(M) is now discussed infurther detail as it relates to tag insertion according to the presentinvention. The modulated video carrier 18 _(M) generates a firsttelevision signal and provides that signal to the tag insertionarrangement 20. The tag insertion arrangement 20 then identifies theportions of the television signal, and particularly, the televisionbaseband signal, in which control information is present. In theexemplary embodiment described herein, the tag insertion arrangement 20determines the portions of the television signal in which the verticalinterval is present.

Contemporaneously, the tag insertion arrangement 20 generates a uniquetagging signal, preferably consisting of a low frequency oscillatingsignal. In the exemplary embodiment described herein, the tagging signalis a 25 Hz sine wave signal. The tag insertion arrangement 20 thenmodulates the tagging signal onto a carrier signal. The carrier signalhas a frequency within the vestigial sideband of the first televisionsignal, and preferably, within 0.9 MHz to 1.15 MHz below the CATVchannel frequency.

The tag insertion arrangement 20 then inserts the tagging signal, whichhas been modulated onto the carrier signal, into the television signalonly during the portions of the television signal in which controlinformation is present. Specifically, the tag insertion arrangement 20inserts the tagging signal in a repeating pattern that corresponds tothe predictable and repeating occurrence of one of the types of controlinformation in the television signal. In the example described herein,the tag insertion arrangement 20 inserts the tagging signal in arepeating pattern that corresponds to the repeating occurrence of thevertical interval in the television signal.

As a result, the tag insertion arrangement 20 of the exemplaryembodiment described above generates a pulsed RF tagging signal. Inparticular, the modulation of the low frequency tagging signal onto thecarrier signal creates an RF tagging signal, and the controllableinsertion of the tagging signal into the television signal during onlythe repeating control information portions of the television signalcreates a pulsed RF tagging signal. The frequency with which the taggingsignal is inserted or pulsed is referred to as the pulse frequency.Thus, in the embodiment described herein, the pulse frequency is equalto the frequency with which the vertical interval occurs, or 60 Hz.

It will be noted that the low frequency tagging signal is preferablychosen to be below one-half the pulse frequency. Specifically, as willbe discussed below, a preferred method of detecting the tagging signalis to extract the tagging signal frequency component from the pulsed RFtagging signal. Because the tagging signal is only present the pulses ofthe pulsed RF tagging signal, the tagging signal must be detectable fromthose pulses. Accordingly, to be able to detect the tagging signal, thefrequency of the tagging signal should be below the Nyquist frequencydefined with respect to the pulse frequency. In the example describedherein, wherein the tagging signal is inserted only during the verticalinterval of the television signal, which occurs at a frequency of about60 Hz, the tagging signal preferably has a frequency of less than 30 Hz.

Moreover, the use of extremely low tagging signal frequencies, such asthose significantly below 20 Hz, tends to increase the possibility ofinterference from multipath signals. In particular, leakage detection isoften carried out by a technician in moving vehicle. It has beenobserved that moving vehicles are particularly susceptible to multipathsignals, which can create, from the vehicle's point of view, anamplitude modulated signal having a frequency below 20 Hz.

As a result, the tagging signal is preferably below one-half the pulsefrequency and at least 20 Hz. Accordingly, in the embodiment describedherein, the tagging signal preferably is a low frequency oscillatingsignal having a frequency between 20 Hz and 29 Hz.

The tag insertion arrangement 20 then provides the first televisionsignal and pulsed RF tagging signal to the combiner 24. The combiner 24then combines those signals with the television signals from the othermodulated video sources 18 ₁ . . . 18 _(M) to produce a broadband CATVsignal for transmission. The combiner 24 provides the broadband CATVsignal to the cable 26.

The CATV signal traverses the cable 26 to the splitter 28. The splitter28 then distributes the CATV signal to each of the television receivers16 ₁ . . . 16 _(N). One or more of the television receivers 16 ₁ . . .16 _(N) tunes to a select CATV channel and performs the appropriatesignal processing to provide a visible and audible presentation of theprogram information.

The pulsed RF tagging signal does not significantly interfere, norperceptively interfere, with the visible and audible presentation of theprogram information in the first television signal or any othertelevision signal. In particular, standard IF filters within thetelevision receivers 16 ₁ . . . 16 _(N) substantially attenuate thevestigial sideband of the CATV channels to which they are tuned.Accordingly, the IF filter of any television receiver tuned to receivethe first television signal would essentially filter out the pulsed RFtagging signal.

Moreover, the synchronization of the insertion of the tagging signalwith the control information portions of the first television signalensure that any potential low level interference caused by the taggingsignal only affects the non-program information portions. As a result,the video and audio information is not adversely affected by the taggingsignal.

It will be understood that other embodiments of the present inventionare possible that employ only one of the above-described interferencereduction features. One such embodiment employs the technique ofcontrollably inserting the tagging signal exclusively during controlinformation portions of the television signal, whereby the taggingsignal is inserted by modulating the tagging signal directly onto thetelevision signal itself. In such a case, the generation of a vestigialsideband carrier signal is not necessary. Such an embodiment isdescribed in further detail below in connection with FIG. 6. While suchan embodiment would ostensibly reduce component cost by eliminating theneed to generate a vestigial sideband carrier signal, the possibility ofinterference with the picture and/or sound quality perceived by the enduser may undesirably increase.

Alternatively, the use of the vestigial sideband may sufficiently reducethe possibility of interference with picture and sound quality such thatthe tagging signal need not be inserted exclusively during the controlinformation portions of the television signal. Accordingly, the taggingsignal could be modulated onto the vestigial sideband carrier signal andcombined continuously with the first television signal. As above,although continuous insertion of the RF tagging signal may reducecomponent cost, the possibility of perceptible interference mayincrease.

FIG. 2 shows a block diagram of an exemplary embodiment of the taginsertion arrangement 20 of FIG. 1. The tag insertion arrangement 20 isoperable to generate a pulsed RF tagging signal as described above inconnection with FIG. 1, and insert the pulsed RF tagging signal into atelevision signal of a CATV channel. As discussed above, the pulsed RFtagging signal is detectable by corresponding leakage detectionequipment and introduces little or no degradation to the image and audioinformation perceived by the subscriber.

While the pulsed RF tagging signal is substantially transparent orundetectable by the end user, the leakage detector 21 readily detectsthe RF tagging signal if placed in the vicinity of leakage in thedistribution system 14. In leakage detection operation, a fieldtechnician moves the leakage detector 21 along various portions of thedistribution system 14 to attempt to identify sources of leakage. Theleakage detector 21 is a device that detects RF signals within thefrequency band of the pulsed RF tagging signal and is further operableto identify the distinctive low frequency tagging signal that has beenmodulated onto the carrier signal and pulsed in synchronization with thecontrol information portions of a standard television signal.

If the leakage detector 21 detects sufficient signal energy within theappropriate signal band, and further identifies a substantial lowfrequency tagging signal component in the detected signal energy, thenthe technician may determine the existence and location of a leakagesituation in the distribution system 14. A suitable leakage detectiondevice is described below in connection with FIG. 4.

Accordingly, the present invention, like other tagging arrangements,utilizes a tagging signal to allow the leakage detector to discriminatebetween a signals caused by leakage in the CATV system under test andspurious signals in the same frequency band. However, in contrast to theprior art, the present invention further provides a leakage taggingarrangement which causes little or no disruption of the televisionpicture or sound quality. Specifically, use of the vestigial sideband ofa particular CATV channel reduces the potential for interference withthat (or any other) CATV channel television signal. Additionally, evenif some low level interference into the television signal band didoccur, the insertion of the tagging signal into the television signalonly during the control information portions further reduces any chancethat the tagging signal will affect picture and sound quality.

The tag insertion arrangement 20 of FIG. 2 includes a detector 40, atelevision signal input 41, a synchronization circuit 42, a splitter 43,a gate generator 44, controller 46, a carrier signal generator 48, apulse switch 50, a low frequency tagging signal generator 54, and asignal inserter comprising a variable gain amplifier (“VGA”) 52 and atwo port combiner 53. In the exemplary embodiment described herein, thedetector 40 further comprises a log amplifier detector 56 and anamplifier 58. Alternatively, the detector 40 may comprise another typeamplitude modulation detector, such as a diode detector.

The television signal input 41 is operably connected to receive thefirst television signal from the modulated video source 18 _(M) (seeFIG. 1) and provide that signal to the splitter 43. The splitter 43 isconnected to provide the first television signal to the log amplifierdetector 56 and the two port combiner 53. The log amplifier detector 56is further connected to the amplifier 58. The amplifier 58 is aninverting amplifier that is operably connected to the synchronizercircuit 42.

The synchronizer circuit 42 is a circuit that is operable to detect thevertical interval in a baseband television video signal. In theexemplary embodiment described herein, the LM1881 video sync separatorintegrated circuit available from National Semiconductor Corporation isused as the synchronizer circuit 42. The synchronizer circuit 42includes a vertical sync output 42 a that is connected to the gategenerator 44. The gate generator 44 is a circuit operable to receivesignals from the vertical sync output of a synchronizer circuit such asthe synchronizer circuit 42 and generate a gate pulse signal therefrom.The gate pulse signal is a pulse signal that comprises pulses that aresubstantially synchronous with the vertical interval of the firsttelevision signal.

The gate generator 44 is connected to a control input 50 a of the pulseswitch 50. The pulse switch 50 further includes an RF input 50 b and anRF output 50 c. The pulse switch 50 is a device or circuit that operatesas a controllable switch with a high degree of isolation. The pulseswitch 50 in the exemplary embodiment described herein comprises an RFswitch 60 and a switchable amplifier 62, each having a control inputconnected to the control input 50 a. The use of two devices, in otherwords, the RF switch 60 and the switchable amplifier 62, enables thepulse switch 50 to provide a higher degree of isolation. It will beunderstood that more or less isolation may be provided by increasing ordecreasing, respectively, the number of switching devices.

The controller 46, which may suitably be a microprocessor, is operablyconnected to control the operations of the carrier signal generator 48and the low frequency tagging signal generator 54. The carrier signalgenerator 48 comprises an oscillator circuit that is operable togenerate an RF signal having a frequency within at least some portion ofthe CATV signal bandwidth. In the exemplary embodiment described herein,the carrier signal generator 48 is operable to generate RF signalshaving a frequency between 115 MHz and 140 MHz. The 115 MHz to 140 MHzfrequency band includes the frequency band in which CATV system leakageis typically tested. The carrier signal generator 48 is connected toprovide the generated RF carrier signal to the RF input 50 b of thepulse switch 50.

The RF output 50 c of the pulse switch is operably connected to the VGA52. The VGA 52 includes a control input 52 a connected to the lowfrequency tagging signal generator 54. The VGA 52 is an RF amplifierthat provides amplification at a level corresponding to control signalat its control input 52 a. As such, the VGA 52 may be used to amplitudemodulate the input RF carrier signal. It will be understood that the VGA52 may alternatively be replaced by another amplitude modulation means,such as, for example, a variable attenuator.

The low frequency tagging generator 54 is preferably an oscillatoroperable to generate a 20 Hz to 29 Hz sine wave signal and is connectedto provide that signal to the control input 52 a of the VGA 52. The RFoutput 52 b of the VGA 52 is coupled to the two port combiner 53. Thetwo port combiner 53 is then further connected to combiner 24 of FIG. 1.

Alternatively, the controller 46 may include a square wave oscillatingoutput that is directly connected to the control input 52 a. In such acase, the controller 46 would comprise the low frequency tagginggenerator. One potential drawback to such an approach is that the squarewave produced by the controller 46 is more likely to introduceundesirable noise than a sine wave produced by a sine wave oscillator.

In the operation of the tag insertion arrangement 20 of FIG. 2, the logamplifier detector 56 receives the first television signal from thetelevision signal input 41 through the splitter 43. The first televisionsignal preferably has a CATV channel (carrier) frequency of between 115and 140 MHz. For the purposes of this example, the first televisionsignal is assumed to have a channel frequency of 133.2625 MHz, and isassumed to conform with NTSC standards. The first television signalfurthermore has a signal band from approximately 131.7675 to 137.7625MHz, which includes a video signal band, an audio subcarrier, achrominance subcarrier, and a vestigial sideband. The video signal bandextends from 133.2625 MHz to approximately 137.4625 MHz. The audiosubcarrier is located at 137.7625 MHz and the chrominance subcarrier islocated at 136.8425 MHz. The vestigial sideband extends fromapproximately 131.7675 MHz to approximately 133.2625 MHz. It is notedthat the lower limit of vestigial sideband is defined by the location ofthe audio subcarrier frequency of the adjacent CATV channel, which islocated at approximately 131.7675 MHz. The video signal band, the audiosubcarrier, and the chrominance subcarrier are collectively referred toas the active sideband.

From the first television signal, the log amplifier detector 56generates a signal that is representative of the logarithmic value ofthe amplitude modulation of the first television signal. In other words,because only the video signal (as opposed to the audio signal andchrominance signal) is amplitude modulated, the log amplifier detector56 essentially generates a form of the video baseband signal. Suchdetectors are well known. The log amplifier detector 56 provides thesignal to the amplifier 58. The amplifier 58 amplifies and inverts thelog amplifier detector output signal to convert it into the properformat for use by the synchronization circuit 42. The converted signalis referred to herein as the modified television baseband signal.

The synchronization circuit 42 receives the modified television basebandsignal from the inverting amplifier 58 and generates a vertical syncoutput signal therefrom. To this end, the synchronization circuit 42provides a vertical sync output signal pulse that is approximatelysynchronous with the first serration pulse of each vertical interval ofthe first television signal. The vertical sync output signal pulse has aduration that is typically less than the duration of the verticalinterval.

FIG. 3 shows a signal timing diagram of the vertical interval of astandard NTSC baseband television signal. The vertical interval includesa plurality of equalization pulses 92 followed by a plurality ofserration pulses 94, followed by another plurality of equalizationpulses 96. The synchronization circuit 42 of FIG. 2 generates thevertical sync output signal pulse upon the detection of the transitionlow 98 of the first serration pulse. The vertical sync output signalpulse has a duration of less than the entire vertical interval.

Referring again to FIG. 2, the synchronization circuit 42 provides thevertical sync output signal to the gate generator 44. The gate generator44 then uses the vertical sync output signal and a priori knowledgeabout standard baseband television signal format to provide a gatesignal having a pulse that begins at approximately the beginning of thenext occurring vertical interval and has a duration approximatelyequivalent to the vertical interval. A gate generator circuit havingsuch capabilities may readily be implemented by those of ordinary skillin the art, and may include, for example, one or more one-shot deviceshaving timing characteristics that correspond to the standard televisionvideo signal format.

The gate signal is then provided to the control input 50 a of the pulseswitch. The gate signal “on” pulse causes the switchable amplifier 62 toturn on and the RF switch 64 to close. Meanwhile, the controller 46causes the carrier signal generator 48 to generate a carrier signalhaving a frequency within the vestigial sideband of the first televisionsignal. Preferably, the controller 46 causes the carrier signalgenerator 48 to generate a carrier signal having a frequency 0.9 MHz to1.15 MHz below the CATV channel frequency of the television signal.Accordingly, in the example discussed herein, the carrier signalgenerator 48 generates a carrier signal having a frequency between132.1125 MHz and 132.3625 MHz. Such frequencies are both sufficientlybelow the video signal band of the first television signal andsufficiently above the audio subcarrier of the adjacent CATV channel soas to reduce the potential for noticeable interference to the end userstuned to either CATV channel.

In any event, the pulse switch 50 receives the carrier signal at the RFinput 50 b and provides the carrier signal at its RF output 50 c onlywhen the switchable amplifier 62 is turned on and the RF switch 64 isclosed. Accordingly, the pulse switch 50 provides the carrier signal tothe RF output 50 c synchronous to the vertical interval of the firsttelevision signal.

During the gate signal pulse, the carrier signal propagates to the VGA52, which then amplitude modulates the carrier signal with the taggingsignal produced by the low frequency tagging signal generator 54. Inparticular, VGA 52 provides an amplitude modulation which may beexpressed as A cos(2πf_(tag)t) where A is the tag amplitude and f_(tag)is the frequency of the tagging signal. Preferably, the tagging signalis approximately 3 dB depth modulated onto the carrier signal. Theamplitude modulated carrier signal is then provided to the two portcombiner 53, where it is added to the first television signal receivedfrom the splitter 43.

When the vertical interval is not present, the gate generator 44provides no pulse to the pulse switch 50. As a result, the carriersignal generated by the carrier signal generator 48 is not provided tothe pulse switch output 50 c. The VGA 52 thus does not receive ormodulate any carrier signal, and no signal is inserted into televisionsignal.

The tagging signal is thus inserted into the television signal onlyduring the vertical interval of the television signal. The two portcombiner 53 thus produces as an output a pulsed RF tagging signal andthe first television signal.

It will be noted that the carrier signal generator 48, the pulse switch50, and the VGA 52 are selected such that the pulsed RF tagging signalhas a predetermined peak power level, preferably on the order of 5 dBbelow the peak power level of the first television signal.

In any event, the two port combiner 53 provides the combined televisionsignal and pulsed RF tagging signal to the combiner 24 of thetransmission subsystem 12 of FIG. 1. As discussed above in connectionwith FIG. 1, the pulsed RF tagging signal is thereafter transmitted overthe CATV distribution system.

The tagging signal is detectable by a suitable leakage detector, butshould not noticeably affect video or audio signal quality in thedestination point television receivers. In particular, the IF filters ofsubstantially all commercially available televisions will sufficientlyattenuate the vestigial sideband of each television signal, therebysubstantially attenuating the carrier signal modulated with the taggingsignal. Moreover, any vestige of the tagging signal that is notattenuated by the television IF filter will only appear in the verticalinterval of the first television signal. Because the vertical intervalof any television signal is not user-viewable (or audible) program data,the pulsed tagging signal will not affect the perceivable video and/oraudio program content.

FIG. 4 shows a circuit block diagram of an exemplary leakage detector300 operable to detect and measure a pulsed RF tagging signal such asthat produced by the tag insertion arrangement 20 described above inconnection with FIGS. 1 and 2. More particularly, the leakage detector300 is operable to detect and measure leakage signals in a televisionsignal distribution subsystem, using the distinctive tagging signal todiscriminate between leakage signals of the system under test andspurious RF signals generated by other sources. To this end, the leakagedetector 300 measures the energy level or signal strength at thefrequency on which the pulsed RF tagging signal has been modulated, andthen determines whether the distinctive tagging signal is present.

The leakage detector 300 includes an RF receiver circuit 302, an analogto digital (“A/D”) converter 304, a digital filter circuit 306, a pulseextraction circuit 310, a tag signal extraction circuit 312, and anoutput circuit 314.

For the purposes of describing the leakage detector 300, it is assumedthat the pulsed RF tagging signal comprises a 25 Hz oscillating signalmodulated onto an RF carrier signal of 132.2625 MHz, which is within thevestigial sideband of the CATV channel at 133.2625 MHz. The pulsed RFtagging signal is furthermore pulsed or inserted into the televisionsignal on the CATV channel in synchronization with the verticalintervals of the television signal. The television signal is assumed tohave a standard NTSC television signal format. As a result, the pulsesof the pulsed RF tagging signal appear at a frequency of approximately60 Hz, which is the frequency of the vertical interval.

Referring again to FIG. 4, the RF receiver circuit 312 is a circuitoperable to receive the pulsed RF tagging signal and perform frequencyconversion and filtering thereon to produce an intermediate frequency(“IF”) signal. In the exemplary embodiment described herein, the IFsignal preferably has a 93 kHz bandwidth centered at 75 kHz. To thisend, the RF receiver circuit preferably includes a front end receiver316 and a band pass filter 318. The front end receiver 316 includesfront end filtering, amplification and frequency conversion circuitryoperable to receive the broadband CATV signal and produce an IF signalcomprising the pulsed RF tagging signal centered at 75 KHz.

A suitable front end receiver circuit 316 and band pass filter 318 wouldbe known to those of ordinary skill in the art. For example, U.S. patentapplication Ser. No. 08/767,991, which is assigned to the assignee ofthe present invention and incorporated herein by reference, describessuch a suitable front end receiver circuit and corresponding band passfilter.

The A/D converter 304 receives the IF signal and generates a digital IFsignal therefrom. In the embodiment described herein, the A/D converter304 has a sampling rate of 1 MHz. The digital IF signal comprises asampled version of the frequency converted pulsed RF tagging signal.

The digital input circuit 306 then performs demodulation and decimationon the digital IF signal to produce a digital baseband signal. Thedigital baseband signal, as a result of the decimation, has an effectivesampling rate of approximate 3900 Hz. The digital baseband signal is adigital representation of the pulsed tagging signal, with pulsesoccurring at a frequency of about 60 Hz.

In particular, the digital input circuit 306 comprises the followingfunctional blocks: an absolute value block 320, averaging block 322,moving average block 324, and buffer 326. The above-referencedfunctional blocks are digital signal processing blocks described hereinin terms of their function. It shall be noted that the operations of thevarious functional blocks may suitably be carried out through a digitalsignal processor, one or more field programmable gate arrays, discretecomponents, or a combination thereof.

In any event, the absolute value block 320 operates to obtain theabsolute value of each sample of the input 1 MHz digital IF signal. Theabsolute value block 320 then provides the absolute value samples to theaveraging block 322. The averaging block 322 takes a block average ofeach successive set of 256 adjacent absolute value samples and producesan output sample consisting of the average value for that set ofsamples. For example, the averaging block 322 receives sample numbers0-255 and produces a first output sample having the average value ofthose samples, then receives sample numbers 256-511 and produces asecond output sample having the average value of those samples, and soforth.

As a result, the averaging block 322 produces one output sample forevery 256 input samples, or one output sample for every 256 μ-secs. Theblock averaging function of the averaging block 322 thus provides thedecimation and the demodulation functions of the digital input circuit306.

The averaging block 322 then provides the output samples to the movingaverage block 324. The moving average block 324 generates a runningaverage of every two output samples from the averaging block 322. Forexample, the moving average block 324 takes the average of samples 1 and2 from the averaging block 322, then takes the average of samples 2 and3 from the averaging block 322, and so forth. As a result, the movingaverage block 324 effectively produces a sample for every sample itreceives from the averaging block 322. The operation of the movingaverage block 324 provides additional filtering to the decimated anddemodulated digital IF signal.

The moving average block 324 provides the moving average output samplesto the buffer 326. The buffer 326 stores samples for several pulseperiods, where pulse period is the time between two pulses in the pulsedRF tagging signal. Accordingly, in the example described herein, thepulse period is {fraction (1/60)}th of a second. The buffer 326preferably stores samples corresponding to an entire second, or 60 pulseperiods.

The absolute value, averaging and moving averaging functions of thedigital input circuit 306 thus operate to decimate and demodulate thedigital IF signal, thereby producing the digital baseband signal. Asdiscussed above, the digital baseband signal comprises samples having aneffective sampling rate of approximately 3.9 kHz which provide a digitalrepresentation of the pulsed tagging signal.

The pulse extraction circuit 310 then receives the digital basebandsignal and generates a digital pulse signal therefrom. The resultingdigital pulse signal comprises a series of digital samples in which eachsample represents the peak value of each pulse in the digital basebandsignal. Specifically, the pulse extraction circuit 310 essentiallyextracts one digital baseband signal sample for each pulse. Thus,because the digital baseband signal comprises a digital representationof a pulsed tagging signal having pulse frequency of approximately 60Hz, the pulse extraction circuit 310 generates a digital pulse signalconsisting of a 60 sample per second signal.

FIG. 5 shows an exemplary pulse extraction circuit 400 which may be usedas the pulse extraction circuit 310 of FIG. 4. The pulse extractioncircuit 400 includes a cross correlation block 402, a timing vector 404,a pulse phase block 406 and a peak output block 408. Those of ordinaryskill in the art may readily implement the various functional blocks ofthe pulse extraction circuit 400 using discrete components, aprogrammable digital signal processing circuit, or the like.

The timing vector 404 comprises a series of N binary samples, where N isthe number of digital baseband signal samples stored in the buffer 326.In the example described herein, N is the number of samples in onesecond, which is approximately equal to 3910. Within the series of Nbinary samples, every Mth sample has a value of “1” while all othersamples have a value of “0”, where M is the number of samples in a pulseperiod. Accordingly, the timing vector is essentially a series of binarysamples that have a “0” value except for one sample that has a “1” valuethat appears every {fraction (1/60)}th of a second. The timing vector404 provides such samples to the cross correlation block 402 and thepeak output block 408.

Contemporaneously, the cross correlation block 402 receives the digitalbaseband signal in blocks of N samples, in other words, one second'sworth of samples, from the buffer 326 of the digital input circuit 306.The cross-correlation block 402 performs a cross correlation between thedigital baseband signal samples and the timing vector samples togenerate a series of values from which the pulses may be identified. Thecross-correlation with the timing vector is necessary because thedigital baseband signal has a relatively low signal to noise ratio.

To carry out the cross correlation, the cross correlation blockgenerates cross correlation values, CX(m), for m=0 to M−1 using thefollowing equation:

CX(m)=Σs(n)*V(n+m), for n=0 to N−1,

where s(n) is the nth sample of the N digital baseband signal samplesreceived from the buffer 326, and V(n+m) is the (n+m)th sample of thetiming vector 404.

The cross-correlation block 402 provides the CX(m) values to the pulsedelay block 406. The pulse delay block identifies the maximum CX(m)value, and provides the m-value of that maximum to the peak output block408. The m-value represents the phase delay between the “1” samples inthe timing vector 404 and the pulses in the digital baseband signal.

The peak output block 408 also receives the N digital baseband signalsamples and the timing vector 404. Using the timing vector 404 for pulsefrequency information, and the m-value as the pulse phase information,the peak output block 408 provides as output a single sample from thedigital baseband signal for each pulse occuring therein. As a result,the peak output block 408 generates a digital pulse signal comprisingN/M output samples for each N digital baseband signal samples received.As mentioned above, in the example described herein, the digital pulsesignal comprises 60 samples per second.

The resulting digital pulse signal is then provided to the tag signalextraction circuit 312. In general, the tag signal extraction circuit312 generates two outputs. The first output is a measurement of thepulse peak amplitude, PP. The pulse peak amplitude is representative ofthe signal strength of the pulsed RF tagging signal detected at the RFreceiver circuit 30. The second output is the f_(tag) component relativeto the pulse peak amplitude, or simply relative f_(tag) component, wheref_(tag) is the frequency of the inserted tagging signal. The relativef_(tag) component is representative of the tag amplitude, A, generatedwithin the tag insertion arrangement 20 of FIG. 2. In general, the pulsepeak amplitude generally describes the strength of the signal detectedat the relevant RF frequency, while the relative f_(tag) componentidentifies whether that signal strength was generated by a pulsed RFtagging signal generated according to the present invention.

Accordingly, in the example described herein, the tag extraction circuit312 generates a pulse peak amplitude value and a relative 25 Hzcomponent value. Because the tagging signal is 3 dB depth-modulated ontothe pulsed RF carrier signal, as discussed above, the relative 25 Hzcomponent should be approximately 0.17. It is noted however, that if thedetected pulse peak amplitude had been generated by spurious signals andnot by leakage signals, then the relative 25 Hz component would besubstantially lower.

To carry out the above described functions, the tag extraction circuit312 preferably includes an input buffer 328, a correlation block 330, amean block 332, a division block 334 and a summation block 335. Theinput buffer 328 receives the digital pulse signal and provides thatsignal to the correlation block 330 and the mean block 332. Thecorrelation block 330 then generates a measurement of the relativef_(tag) component of the digital pulse signal. In this embodiment, thecorrelation block 330 generates a value equal to A*PP, where A is therelative f_(tag) component and PP is the pulse peak component.

To this end, the correlation block 330 carries out the followingequation using the appropriate digital signal processing functions:

PP*A=[(2/N)Σ{P(n)cos(2πf _(tag) Tn)}]²+[(2/N)Σ{P(n)sin(2πf _(tag) Tn)}]²

where n is a sample index, P(n) is the nth digital pulse signal samplein one second, T is the pulse period, and N is the number of digitalpulse signal samples in one second.

Contemporaneously, the input buffer 328 also provides the digital pulsesignal to the mean block 332. The mean block 332 generates the mean overN digital pulse signal samples. It can be shown that the resulting meanblock output value is equal to the PP*(1−A). The division node 334 thendivides the correlation block output value by the mean block outputvalue in order to factor out the PP value from the correlation blockoutput value. The resulting value, which is referred to as the f_(tag)measurement value, is A/(1−A).

The summation node 335 receives and sums the mean block output valuewith the correlation block output value. The resulting value isPP*A+PP*(1−A), or simply, PP. The value PP is the measured pulse peakamplitude. The pulse peak amplitude is a measure of the power or signalstrength detected at 332.2625 MHz, regardless of whether that signalstrength was attributable to the pulsed RF tagging signal.

The output circuit 314 then receives the f_(tag) measurement value andthe pulse peak amplitude from the tag extraction circuit 312 andgenerates either a visible and/or audible indication therefrom. Forexample, the output circuit 314 may include a visible display thatdisplays information indicative of the pulse peak amplitude and anotherindicator, either audible or visible, that communicates whether thetagging signal was detected.

To this end, the output circuit 314 may suitably include a thresholdblock 336 and an audible or visible indication circuit 338. Thethreshold block 336 receives the f_(tag) measurement value from thedivision node 334 and determines whether the f_(tag) measurement valueis within a certain range that is consistent with the depth modulationof the tagging signal as generated by the tagging signal arrangement 20of FIG. 1. In the example described herein, wherein the tagging signalis 3 dB depth-modulated, the threshold block determines whether thef_(tag) measurement value is within a predetermined range of 0.17, suchas, for example from 0.08 to 0.26. If so, then the threshold block 336determines that the detected signal energy, as represented by the pulsepeak amplitude, is attributable to the pulsed RF tagging signal, andprovides an indicator signal to the indication circuit 338. If not, thenthe threshold block 336 determines that pulse peak amplitude is notattributable to the pulsed RF tagging signal and does not provide theindication signal to the indication circuit 338.

In either event, the indication circuit 338 receives the pulse peakamplitude value from the summation node 335 and provides a visibledisplay indicative thereof. In addition, the indication circuit 338provides another visible display and/or an audible display responsive tothe indication signal received from the threshold block 336.

As a result, the technician is provided a display that shows ameasurement of the pulse peak amplitude, which in turn is indicative ofthe signal level energy detected in the frequency band around 132.2625MHz, and an indication of whether that signal level energy isattributable to the pulsed RF tagging signal generated by the taginsertion arrangement 20 within the transmission subsystem 12.

The leakage detector 300 thus provides a means by which leakage signalswhich have been tagged in accordance with the present invention may bedetected. Spurious signals from other cable systems are distinguishedsuch that the leakage detection is truly directed toward leakage fromthe system under test.

FIG. 6 shows an alternative embodiment of the tag insertion arrangementwhich may be used in the CATV system 10 of FIG. 1. In this embodiment,the tagging signal is inserted into the television signal by directamplitude modulation of the first television signal with the taggingsignal. The tag insertion arrangement 80 of FIG. 6 is similar instructure to the tag insertion arrangement 20 of FIG. 2, and devicescommon to both devices will have the same reference numbers.

The tag insertion arrangement 80 comprises a detector 40, a televisionsignal input 41, a synchronization circuit 42, a splitter 43, a gategenerator 44, a low frequency tagging signal generator 54, a taginserter comprising a VGA 52, a DC reference 66 and a two-way switch 64.The television signal input 41 is connected to the splitter 43 and thesplitter 43, is further connected to the detector 40. The splitter 43 isalso directly connected to the VGA 52.

The detector 40 is connected to the synchronization circuit 42, which inturn is connected to the gate generator 44. The detector 40, thesynchronization circuit 42 and the gate generator 44 all have the samestructure and operation as that described above in connection with FIG.2. Analogous to the embodiment of FIG. 2, the gate generator 44 isconnected to a control input of the two-way switch 64. In the presentembodiment, the two-way switch 64 is a two-way analog switch connectedto the control input 52 a of the variable gain amplifier 52. The switch64, under control of the gate generator 44, alternatively connects thecontrol input 52 a of the VGA 52 to either the low frequency taggingsignal generator 54 or the DC reference voltage source 66.

In the operation of the alternative embodiment, the detector 40, thesynchronizer 42 and the gate generator 44 operate as before to generatea gate signal pulse that is substantially synchronous with the verticalinterval of the first television signal. The pulse causes the switch 64to connect the low frequency tagging signal generator 54 to the controlinput 52 a of the VGA 52. The low frequency tagging signal generator 54then drives the VGA 52 with a low frequency oscillation, such as, forexample a 25 Hz signal, thereby causing low frequency modulation of thefirst television signal.

When the pulse is not present, the switch 64 connects the DC referencevoltage source 66 to the control input 52 a. The DC reference voltagesource 66 biases the VGA 52 such that the VGA 52 effectively does notchange the first television signal. With such a configuration, the firsttelevision signal is modulated by the low frequency tagging signal onlyduring the vertical interval.

It will be noted that the above described embodiments of the presentinvention are merely illustrative. Those of ordinary skill in the artmay readily devise their own implementations that incorporate theprinciples of the present invention and fall within the spirit and scopethereof. In particular, it will be appreciated that the presentinvention may readily be implemented in television signals other thanthose having an NTSC standard format, such as for example PAL-formattedtelevision signals. Moreover, the use of the vertical interval as thecontrol information portion of the television signal is given by way ofexample only, and other periodic control information portions of astandard television signal, such as, for example, one or more quietlines, or one or more horizontal sync pulses, may be employed. It isfurthermore noted that specific frequencies, sampling rates, and thelike may readily be altered by those of ordinary skill in the art to fittheir particular implementation needs.

We claim:
 1. A method of detecting leakage in a communication system,the communication system transmitting television signals comprisingcontrol information and program information, the method comprising: a)generating a tagging signal, said signal being detectable bycorresponding leakage detecting equipment; b) identifying a portion of atelevision signal in which control information is present; c) insertingthe tagging signal into the television signal only during the portion ofthe television signal in which control information is present. d)disposing the leakage detecting equipment in a location in which leakageis desired to be measured; e) indicating a presence of leakage on thecommunication system based on the detection of the inserted taggingsignal.
 2. The method of claim 1 wherein step c) further comprisesmodulating a carrier signal with the tagging signal only during theportion of the television signal in which control information is presentand summing the carrier signal modulated with the tagging signal withthe television signal.
 3. The method of claim 1 wherein step c) furthercomprises modulating the television signal with the tagging signal onlyduring the portion of the television signal in which control informationis present.
 4. The method of claim 1 wherein step a) further comprisesgenerating an oscillating tagging signal.
 5. The method of claim 2wherein step c) further comprises modulating a carrier signal with thetagging signal, wherein the carrier signal has a frequency within thevestigial sideband associated with the television signal.
 6. The methodof claim 1 wherein the control information further comprises a verticalinterval and step b) further comprises identifying a portion of atelevision signal in which the vertical interval is present, and step c)further comprises inserting the tagging signal into the televisionsignal only during the portion of the television signal in which thevertical interval is present.
 7. The method of claim 6 wherein step c)further comprises modulating a carrier signal with the tagging signalonly during the portion of the television signal in which the verticalis present and summing the carrier signal modulated with the taggingsignal with the television signal.
 8. The method of claim 1 wherein stepe) further comprises providing a visible indication responsive to thedetection of the inserted tagging signal.
 9. The method of claim 1wherein step e) further comprises providing a visible indication of ameasured signal energy level of the inserted tagging signal.
 10. Anarrangement for the insertion of a tagging signal into a televisionsignal, the television signal comprising control information and programinformation, the television signal being transmitted by a communicationsystem, the arrangement comprising: a) an input connected to a source oftelevision signals comprising control information and programinformation; b) a tagging signal generator operable to generate atagging signal that is detectable by corresponding leakage detectingequipment; and c) a signal inserter, coupled to the input and thetagging signal generator, operable to insert the tagging signal into thetelevision signal; d) a switch operably coupled to the signal inserterto cause the signal inserter to insert the tagging signal only duringthe portion of the television signal in which control information ispresent.
 11. The arrangement of claim 10 further comprises e) a detectorcoupled to the input, the detector operable to identify a portion of thetelevision signal in which control information is present and whereinsaid detector provides a signal to the switch indicative of the presenceof control information in the television signal.
 12. The arrangement ofclaim 10 wherein said signal inserter comprises an amplitude modulatorconnected to the input, said amplitude modulator further comprising anRF output, and a control input connected to the switch.
 13. Thearrangement of claim 10 wherein the tagging signal generator is anoscillator.
 14. The arrangement of claim 13 wherein the oscillator isoperable to generate an oscillating signal having a frequency of lessthan 30 Hz.
 15. The arrangement of claim 13 wherein the switch has aswitching frequency, and wherein the oscillator is operable to generatean oscillating signal that has a frequency of less than one half of theswitching frequency.
 16. The arrangement of claim 10 wherein the controlinformation further comprises a vertical interval and said switch isfurther operable to cause the signal inserter to insert the taggingsignal into the television signal only during the portion of thetelevision signal in which the vertical interval is present.
 17. Anarrangement for performing leakage detection in a communication system,the arrangement comprising: a) an input connected to a source oftelevision signals comprising control information and programinformation; b) a tag insertion circuit operable to generate a taggingsignal; identify a portion of a television signal in which controlinformation is present; and insert the tagging signal into thetelevision signal only during the portion of the television signal inwhich control information is present. c) a leakage detector operable todetect the tagging signal and generate an output responsive to detectionof the tagging signal, the output indicative of a detection of leakagein the communication system.
 18. The arrangement of claim 17 wherein thetag insertion circuit is further operable to modulate the televisionsignal with the tagging signal only during the portion of the televisionsignal in which control information is present.
 19. The arrangement ofclaim 17 wherein the control information further comprises a verticalinterval and the tag insertion circuit is further operable to identify aportion of a television signal in which the vertical interval ispresent, and insert the tagging signal into the television signal onlyduring the portion of the television signal in which the verticalinterval is present.
 20. The arrangement of claim 17 wherein the leakagedetector includes an RF receiving circuit operable to receive thetelevision signal; a detection circuit operable to detect the presenceof the inserted tagging signal in the television signal; and anindication circuit operable to indicate the detected presence of theinserted tagging signal.
 21. The arrangement of claim 20 wherein thedetection circuit includes an analog to digital converter and a digitaldetection circuit.
 22. The arrangement of claim 20 wherein theindication circuit includes a visible display.
 23. The arrangement ofclaim 21 wherein the digital detection circuit includes: a digitalfilter circuit operable to generate a filtered digital signal havingpulses corresponding to the inserted tagging signal, a pulse extractioncircuit operable to generate a digital pulse signal comprising pulsesamples representative of the pulses of the filtered digital signal; anda tag signal extraction circuit operable to generate informationrepresentative of an amplitude of the inserted tagging signal based onthe pulse samples.